1. Field of the Invention
The present invention relates to an information recording apparatus such as a laser beam printer, and more particularly to an information recording apparatus capable of smoothing process on bit map data representing characters or patterns, thereby smoothing the contours of characters or patterns to be printed and thus improving the print quality.
2. Related Background Art
Laser beams printers employing electrophotographic process are recently employed, for example, in the output units of computers or facsimile apparatus, and in so-called digital copying machines for printing image data read from an image scanner.
Such laser beam printers are capable of printing an image for example with a resolving power of 300 dot/inch.
In such case, the character or pattern is represented, as shown in FIG. 4, by black ( ) and white (.smallcircle.) dots to be printed corresponding to a grating of 300 dot/inch. FIG. 4 illustrates the dot pattern of character "a". The resolving power of 300 dot/inch corresponds to a dot pitch of about 85 microns. Since the human vision is generally said to resolve about 20 microns, the contour of character or pattern formed by the dots of the above-mentioned pitch (85 microns) appears staggered, so that the obtained print cannot therefore be considered as of high quality.
In order to over come such drawback, consideration has been given to the following approaches.
A first approach is to simply increase the resolving power (for example to 1200 dot/inch). This method however requires a bit map memory of a capacity of 4.times.4=16 times for representing a same area, so that the apparatus inevitably becomes very expensive.
A second approach is to increase the resolving power in equivalent manner in the main scanning direction or in the main and sub scanning directions by modifying the print data of an object pixel by referring to the data of surrounding pixels, such as through addition of a buffer memory of a limited capacity instead of expanding the capacity of the bit map memory. Processes of this sort are disclosed in the U.S. Pat. Nos. 4,437,122, 4,700,201 and 4,847,641.
Methods disclosed in the U.S. Pat. Nos. 4,437,122 and 4,700,201 are to correct the data of an object pixel to be printed, by referring to said object pixel and surrounding eight pixels. These methods, only having a limited reference area in the surrounding pixels, can identify that the object pixel is a part of a curved line, but are unable to identify the curvature of said curved line. In particular they are incapable of identifying an almost horizontal or vertical contour and are therefore unable to effect optimum correction according to the curvature, so that it is difficult to obtain optimum result from smoothing.
On the other hand, the method disclosed in the U.S. Pat. No. 4,847,641, utilizing reference in a wider area than in the above-mentioned two methods, can also identify the curvature of a curved line in which the object pixel constitutes a part. In this method, however, though the entire reference area is wider, each of the matching patterns is limited to a part of said reference area. For this reason said method is associated with following drawbacks.
Firstly, this method is incapable of identifying whether the object pixel is a part of a binarized halftone image obtained for example by dither method or error dispersion method. This method, therefore, though capable of effective smoothing for a character image, may erroneously effect smoothing on a part of dots constituting a halftone pixel obtained by the dither method or error dispersion method. For example, FIG. 9A is a part of a 4.times.4 dither image. If reference is made to a limited surrounding area for an object pixel 5f, said object pixel is recognized as a part of a character or a pattern and is changed from a white pixel to a pixel with a density. This results in a local variation of image density of a halftone image, eventually giving rise to deterioration of image quality, such as generation of a pseudo contour.
Secondly, this method is incapable of identifying whether the pixel belongs to a concentrated (or complex) image. FIG. 9B shows an example of image composed of a concentrated group of one dot wide lines. In this case, for smoothing each line, a change in dot density is required in pixels marked with(.DELTA.) or (X) in FIG. 9C. As will be apparent from FIG. 9C, each pixel to be subjected to density change will be positioned adjacent or close to another pixel also subjected to density change because of neighboring pixels thereof, and this fact results in a deterioration of the resolving power. Such complex concentration of pixels may appear not only in a highly concentrated line image but also in alphabets and Chinese characters of small sizes. In such case, the object pixel subjected to a change for smoothing may be positioned close to another pixel to be changed for an adjacent image portion, whereby said object pixel (representing an image line or a line of a character) may become less distinguishable from the adjacent pixel. Such phenomenon results in a significant loss in the resolving power of the image around said object pixel, thus providing a blurred image or a moire pattern on the image, and deteriorating the image quality. Furthermore, if a halftone representation is conducted within a pixel for the purpose of smoothing in such concentrated portion of an image, the image density loses reproducibility due to interaction with the nearby pixels, and is easily affected by circumferential conditions such as temperature and humidity. Thus the smoothing effect varies according to such conditions, and the printed character may appear different.
Naturally such drawbacks can be avoided by adopting a sufficiently wide reference area in each matching pattern, so as to identify whether the object pixel belongs to a dither processed image or a concentrated portion of the image, but such method inevitably requires a large circuitry, thus increasing the cost of the apparatus.
Also methods have been disclosed for changing the object pixel depending on the feature of the boundary portion to which said object pixel belongs, for example in the U.S. Pat. No. 4,933,689 and Japanese Patent Unexamined Patent Publication (Kokai) Nos. 61-214661 and 61-214666, but these methods have been associated with a drawback that the dot size varies by the smoothing process.